1. Field of the Invention
The present invention relates to a charged particle beam writing apparatus and a charged particle beam writing method. For example, the present invention relates to a method for calculating an irradiation time of an electron beam in electron beam writing.
2. Description of Related Art
The lithography technique that advances miniaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. For forming a desired circuit pattern on such semiconductor devices, a master or “original” pattern (also called a mask or a reticle) of high accuracy is needed. Thus, the electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for producing such a highly precise master pattern.
FIG. 13 is a schematic diagram explaining operations of a variable shaped electron beam writing or “drawing” apparatus. As shown in the figure, the variable shaped electron beam writing apparatus operates as described below. A first aperture plate 410 has a quadrangular opening 411 for shaping an electron beam 330. A second aperture plate 420 has a variable-shape opening 421 for shaping the electron beam 330 having passed through the opening 411 of the first aperture plate 410 into a desired quadrangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through a part of the variable-shape opening 421 of the second aperture plate 420, and thereby to irradiate a target object or “sample” 340 placed on a stage which continuously moves in one predetermined direction (e.g., the x direction) during the writing. In other words, a quadrangular shape that can pass through both the opening 411 and the variable-shape opening 421 is used for pattern writing in a writing region of the target object 340 on the stage continuously moving in the x direction. This method of forming a given shape by letting beams pass through both the opening 411 of the first aperture plate 410 and the variable-shape opening 421 of the second aperture plate 420 is referred to as a variable shaped beam (VSB) method.
In the electron beam writing described above, the dose of each beam shot is set such that the value of a beam dose at the edge or “end” of a figure is to be the threshold value of a dose for resolving the resist. Generally, it is set such that about half the maximum of an irradiation energy of a shot at the figure edge reaches the threshold value. For calculating a dose, one dose formula is used irrespective of the position of irradiation. Therefore, even when writing a figure which is formed by connecting a plurality of shots, a dose of each shot is set such that about half the maximum of the irradiation energy reaches the threshold value irrespective of whether each shot concerned is at the edge of a figure or not.
On the other hand, along with recent tendency of miniaturization of patterns, the time period of performing writing by the writing apparatus becomes long. Accordingly, it is required to shorten the time period. However, since it needs to enter a calculated dose into the resist in order to properly write a pattern in accordance with the required size, the conventional method has a limit in shortening the writing time.
When performing irradiation based on an incident dose calculated by a conventional dose formula, the dose at each position of all the regions except for a figure edge and for a place on which nothing is written is larger than the threshold value of the resist. In order for each of all the doses at figure edges to be the threshold value of the resist, it is necessary to let each of all the doses in the vicinity of edges of the figure be larger than the threshold value of the resist. However, as to doses in regions sufficiently distant from the edges of the figure, it is sufficient for each of them to be about the threshold value. This subject has not been taken into consideration in the conventional method. Therefore, for example, in the case of writing a figure formed by connecting a plurality of shots, if an incident dose of a region inside the figure away from the figure edge by a sufficient distance longer than the radius of forward scattering of the beam is calculated by using the conventional method, the dose of the region is larger than the threshold value of the resist. That is, when a dose is large, the irradiation time becomes long in accordance with the dose. Thus, an excessive dose exists depending on a figure or its irradiation position, and accordingly, there is a problem of taking a writing time longer than needed because of such excessive dose.
Then, the inventor of the present invention developed a method of calculating, for each of a plurality of mesh regions made by virtually dividing the writing region of a target object, a dose of an electron beam shot in a mesh region concerned by using a dose formula selected for each mesh region from a plurality of dose formulas, and has already filed the invention (refer to Japanese Patent Application Laid-open (JP-A) No. 2011-228503) prior to the present invention. In this method according to the prior application, in order to correct a proximity effect, for example, a formula is selected for each of mesh regions made by dividing the write region into mesh regions each having a size larger than an influence radius of forward scattering. However, by further research and development by the inventor, it has been found that even a shot in a region inside a figure may be recognized to be at the edge of the figure, depending on a figure size. That is, there is a case where the method does not function sufficiently well.
Such a problem occurs when a figure size is smaller than the size of a divided mesh region. Then, consideration of making the mesh region size small is performed. However, if the size of mesh regions becomes small, the number of mesh regions increases, which causes a new problem that the computation amount becomes large.